On the positive side, access is usually easy, a roof already exists, lighting and possibly electrical outlets are already installed, and it will be easy to provide both egress and ventilation. Utilities are often available. Exterior walls are already in place, often including windows. The exterior walls and ceiling might even be insulated. You will probably need to construct/reconstruct only one exterior wall.
On the negative side, you must consider where you will find space for ways in which the garage is currently being used. For example, where will your workbench and the gardening supply cabinet be relocated? Where will you move the items stored in the garage? You must also consider how the resale value of your home will be affected when there is no enclosed parking.
You will need to either construct a sub-floor level with the rest of the house or alter the steps to make them appropriate for indoors. You will also have at least one exterior wall to construct.